1. Field of the Invention
The present invention relates to a carrier for electrophotography which composes a two-component type developer used to develop an electrostatic image in electrophotography, electrostatic recording or electrostatic printing. It also relates to a two-component type developer making use of the carrier, and an image forming method carried out using the two-component type developer.
2. Related Background Art
It is conventionally known to form an image on the surface of a photoconductive material by an electrostatic means.
A large number of methods are known as electro-photography, as disclosed in U.S. Pat. No. 2,297,691 Japanese Patent Publications No. 42-23910 and No. 43-24748 and so forth. In general, an electrostatic latent image is formed on a photosensitive member, utilizing a photoconductive material and according to various means, and subsequently a very finely divided electrodetective material called a toner is made to adhere to the latent image to form a toner image corresponding to the electrostatic latent image.
Next, the toner image is transferred to an image holding medium such as paper if necessary, followed by fixing by the action of heat, pressure, or solvent vapor. A copy is thus obtained. When the process comprises a toner-image transfer step, the process is usually provided with the step of removing the toner remaining on the photosensitive member.
As developing methods by which the electrostatic latent image is formed into a visible image by the use of a toner, known methods can be exemplified by the powder cloud development as disclosed in U.S. Pat. No. 2,221,776, the cascade development as disclosed in U.S. Pat. No. 2,618,552, the magnetic brush development as disclosed in U.S. Pat. No. 2,874,063, and the method in which a conductive magnetic toner is used, as disclosed in U.S. Pat. No. 3,909,258, as well as what is called the J/B development as disclosed in Japanese Patent Application Laid-open No. 62-63970, in which a bias electric field comprised of an AC component and a DC component is applied across a developer carrying member (a developing sleeve) and a photoconductive layer to carry out development.
Among these, the magnetic brush development can be noted as a representative process. In this process, magnetic particles such as steel powder or ferrite powder are used as a carrier, and a developer comprised of a toner and such a magnetic carrier is held with a magnet so that the developer is arranged in the form of a brush by the action of a magnetic field of the magnet. The magnetic brush thus formed is brought into contact with the electrostatic latent image surface on a photoconductive layer, whereupon only the toner is attracted toward the electrostatic latent image from the brush to carry out development.
The carrier that composes such a two-component type developer used in these development processes can be roughly grouped into a conductive carrier and an insulative carrier. The conductive carrier is usually comprised of oxidized or unoxidized iron powder. A developer composed of this iron powder carrier, however, has the problems that the triboelectric chargeability to a toner is unstable and also that fog may be generated on a visible image formed using the developer. More specifically, as the developer is used, toner particles adhere to and accumulate (spent toner) on the surfaces of the iron powder carrier particles, so that the electrical resistance of carrier particles increases to lower bias currents, and also to make the triboelectric chargeability unstable, resulting in a lowering of the image density of a visible image formed and an increase of fog. Thus, when copies are continuously taken by an electrophotographic copying machine using the developer containing the iron powder carrier, the developer may deteriorate upon copying several times and hence it becomes necessary to change the developer at an early stage, resulting in a high cost after all.
The insulative carrier is commonly typified by a carrier comprising carrier core particles comprised of a ferromagnetic material such as iron, nickel or ferrite whose surfaces are uniformly coated with an insulating resin. A developer that employs this carrier may cause little the melt-adhesion of toner particles to the carrier surfaces, compared with the case of the conductive carrier, and at the same time the triboelectric chargeability of a carrier to a toner can be controlled with ease. Hence, the developer has the advantage that it is suitable particularly for high-speed electrophotographic copying machines in view of its superior durability and long lifetime.
There are various performances required for the insulative carrier. Particularly important performances can be set out as proper chargeability, impact resistance, wear resistance, a good adhesion between cores and coating materials, and uniformity in charge distribution.
Taking account of the above required performances, insulative carriers hitherto used still leave room for improvements, and no perfect carrier is known at present. For example, use of an acrylic resin as a coating material for carriers is disclosed in Japanese Patent Applications Laid-open No. 47-13954 and No. 60-208765. In particular, its molecular weight configuration is disclosed in Japanese Patent Application Laid-open No. 60-208767, and it is known to constantly control molecular weights so that the chargeability of coated carriers can be stabilized. However, making a coating resin adhere to carrier cores tends to be affected by conditions set in apparatus and environments where the carrier cores are coated, in particular, by humidity. Even if these have been severely controlled, no satisfactory measures are available for making the resin stably adhere to cores to achieve sufficient chargeability and durability. This is the present situation of the matter.
Meanwhile, in order to prevent carrier spent such as melt-adhesion of toner, it is proposed to use as a coating material a resin having a small surface energy so as to improve durability, and the resin having a small surface energy is exemplified by silicone resin.
The silicone resin is advantageous in that it has not only a low surface tension but also a high water repellency. On the other hand, the silicone resin is problematic in that it has so poor an adhesion that coat layers formed tend to separate.
In order to overcome such a problem, proposals are made as exemplified by a method in which a resin-modified silicone resin is used (Japanese Patent Application Laid-open No. 55-127569), a method in which vinyl silane is incorporated to cause it to react with other resin (Japanese Patent Application Laid-open No. 56-32149), a method in which a mixture of a trialkoxysilane and ethyl cellulose is used (U.S. Pat. No. 3,840,464) and a method in which a mixture of an organosilicone terpolymer and a polyphenylene resin is used (U.S. Pat. No. 3,849,127). There, however, have been the problems that a high temperature of 300.degree. C. or above is required to form its coat layers, and the silicone resin has so poor a compatibility with other resins that the coating layers become non-uniform and no expected performance can be obtained. It is also proposed to prepare coat layers at a relatively low curing temperature (Japanese Patent Application Laid-open No. 55-127569). Such coat layers, however, tend to wear because of their unsatisfactory adhesion and unsatisfactory toughness. For example, in the course of strong and long-term agitation of developer in a developing assembly of a high-speed copying machine, where carrier particles collide against the inner wall of the developing assembly and the surface of a photosensitive member, the silicone resin coat layers wear and separate to cause a change in triboelectric charging from the charging between toner and silicone resin to the charging between toner and carrier cores, so that the quantity of charges of the developer can not be kept constant, causing a deterioration of image quality.
In recent years, a rapid progress is being made from monochromatic copying to full-color copying, and researches are made on two-color copying machines or full-color copying machines, which have been already put into practical use. For example, Journal of Electrophotographic Society, Vol. 22, No. 1, pp. 7-16, (1983) and Journal of Electrophotographic Society, Vol. 25, No. 1, p.52 (1986) make reports relating to color reproduction and gradation reproduction.
Images formed by full-color electrophotography presently put into practical use, however, are not necessarily satisfactory for those who are accustomed to seeing color pictures that are by no means immediately compared with the actual object or original and also processed more beautifully than the actual object or original, as in television pictures, photographs and color prints.
In full-color electrophotography, color images are commonly formed using color toners of yellow, magenta and cyan three colors to reproduce all colors.
Its process comprises the step of forming an electrostatic latent image on a photoconductive layer through a color-separating light-transmitting filter having the relation of complementary color to the color of a toner, followed by developing and transfer steps, through which a toner image is held on an image holding medium. These steps are repeated several times to superimpose toner images on the same medium while making registration, followed by fixing carried out once to obtain a final full-color image.
In general, in the case of development carried out using the developer of what is called a two-component system comprised of a toner and a carrier, the toner is electrostatically charged to the desired charge quantity and charge polarity by its friction with the carrier, and the electrostatic attraction force produced is utilized to develop electrostatic images. Accordingly, in order to obtain good visible images, the triboelectric chargeability of toner that mainly depends on the relation with the carrier must be kept good.
It is also an important point how the carrier is prevented from deterioration in long-term use. After copies have been taken for a long term, the carrier deteriorates because of deposits formed thereon and changes in properties of its particle surfaces, so that its ability to provide charges to the toner becomes weak to cause toner scatter and faulty images such as fog.
Nowadays, to cope with such problems, a variety of researches are made, e.g, investigations are made on carrier core materials and carrier coat materials, coating weights are made optimum, charge control agents and fluidity-providing agents to be added to toners are studied, and also binders serving as base materials are improved, all of which are made so that a superior triboelectric chargeability can be achieved in the materials that compose developers.
For example, as techniques to add a charging auxiliary such as chargeable fine particles to the toner, Japanese Patent Publication No. 52-32256 and Japanese Patent Application Laid-open No. 56-64352 disclose adding a resin powder having a polarity reverse to the toner, and Japanese Patent Application Laid-open No. 61-160760 discloses adding a fluorine-containing compound, which are added to developers so as to achieve a stable triboelectric chargeability. Nowadays, many charging auxiliaries are also being advanced.
Various measures are also taken as methods for adding such a charging auxiliary. For example, it is common to use a method in which electrostatic attraction force or van der Waals force, acting between toner particles and the charging auxiliary, is utilized to cause the latter to adhere to the toner particle surfaces, where a stirrer, a mixer or the like is used. In such a method, however, it is not easy to uniformly disperse the additive on the toner particle surfaces, and also additive particles not adhering to toner particles may form agglomerates to make it difficult to prevent the presence of additives brought into what is called a free state. This tends to more frequently occur with an increase in specific electrical resistance of the charging auxiliary and with a decrease in particle diameter. In such a case, an influence on the developer may come therefrom. For example, the toner comes to have an insufficient quantity of triboelectricity, resulting in non-uniform image densities and images with much fog.
The above method also have the disadvantage that the content of the charging auxiliary changes when copies are continuously taken, to make it impossible to keep image quality at the initial stage.
As another method for its addition, there is a method in which the charging auxiliary is previously added together with a binder resin and a colorant when toners are produced. However, the amount of the charging auxiliary added or the dispersion thereof on the toner particle surfaces can not be controlled with ease since charge control agents can not be made uniform with ease and also since what substantially contribute to charging performance are charging auxiliaries and charge control agents present in the vicinity of toner particle surfaces and those present inside the particles do not contribute to the charging performance. The quantity of triboelectricity is unstable also in toners obtained by such a method. Thus, as previously stated, under existing circumstances, those having a well satisfactory quality have not been available only if the charging auxiliary is used, like the case when those satisfying developer performances can not be obtained with ease.
Moreover, in recent years, there is an increasing commercial demand for making copying machines achieve greater miniturazation and making images have a higher quality. In the present technical field, it is attempted to make toner particle diameter smaller so that a color image can be formed in a high image quality. Making smaller the particle diameters of toner particles results in an increase in the surface area per unit weight, tending to bring about an excessively large quantity of triboelectricity of the toner. This is accompanied with a possibility of the insufficiency of image density or the deterioration of durability or running performance. In addition, because of the large quantity of triboelectricity, toner particles may strongly adhere to one another to cause a decrease in fluidity, bringing about a problem in the stability of toner feeding and the providing of triboelectricity to the toner.
In the case of color toners, they contain no conductive materials such as magnetic materials and carbon black, and hence have no portions from which charges are leaked to commonly tend to have a larger quantity of triboelectricity. This tendency is more remarkable when polyester type binders having a high charging performance is used.
In particular, color toners are strongly desired to have performances as shown below.
(1) Fixed toners are required to nearly come into a substantially completely molten state to the extent that the forms of toner particles can not be recognized, so as for their color reproduction not to be hindered because of irregular reflection upon exposure to light.
(2) Color toners must have a transparency not to obstruct the toner layer having a different color tone that lies beneath an upper layer thereof.
(3) The respective constituent toners must have well-balanced hues and spectral reflection properties, and sufficient chroma.
From such viewpoints, studies are made on many binder resins. However, none of toners that satisfy all of the above performances have brought out. Nowadays, in the present technical field, resins of a polyester type are widely used as binder resins for color toners. Toners comprised of a polyester resin, however, commonly tend to be affected by temperature and humidity, and tend to cause problems of an excessive charge quantity in an environment of low humidity and an insufficient charge quantity in an environment of high humidity.
Moreover, when some additives are added for the purpose of overcoming such disadvantages, carrier contamination may come into question. This inhibits charging performance of carriers to cause a decrease in charge quantity and cause image fog and toner scatter. Hence, it is sought to design carriers having a high contamination resistance to the additives. Accordingly, as a matter of course, carriers must be so designed as to have a good charging performance, and it is of urgent to make advancement of color toners that can have stable charges over a wide range of environments and also to design carriers for accomplishing color developers having a lifetime long enough to maintain good images over a long period of time as a role of developers.
Meanwhile, in the aforesaid development of electrostatic latent images, a toner with a fine particle diameter is blended with a carrier formed of relatively large particles and is used as a developer for electrophotography. The composition of both the toner and the carrier is selected so that as a result of their mutual contact friction the toner can have a polarity reverse to the charges present on the photoconductive layer. As a result of contact friction between the both, the carrier further electrostatically attracts the toner to its particle surfaces to transport the toner as a developer through a developing assembly and also feed the toner onto the photoconductive layer.
When, however, copies are continuously taken on a large number of copy sheets by an electrophotographic copying apparatus using such a two-component type developer, although sharp images with a good image quality can be obtained at the initial stage, edge effect with much fog may seriously occur after copies have been taken on several tens of thousands of sheets, resulting in images having poor gradation and sharpness.
In color copying carried out using toners with chromatic colors, continuous gradation is an important factor that influences image quality, and the edge effect that stresses only margins of images, occurring after copies have been taken on a large number of copy sheets, greatly damages the gradation of images. For example, quasi-contours due to the edge effect are formed in the vicinity of actual contours, resulting in a loss of reproducibility including color reproducibility in color copying. Image area used in conventional black and white copying is 10% or less and images are almost held by line images as in letters, documents, reports and so forth. On the other hand, in the case of color copying, image area is 20% at least, and images are held by gradational solid images at a reasonable frequency or occupancy as in photographs, catalogues, maps, pictures and so on.
When copies are continuously taken using such originals having a large image area, reproductions with a high image density can be obtained at the initial stage in usual instances, but the feeding of toner to the two-component type developer may become insufficient with time to cause a decrease in density, or the toner being fed and the carrier may mix in the state of charge insufficiency to cause fog or cause a local increase or decrease in toner concentration (which indicates toner-carrier mixing ratio) on the developing sleeve, tending to result in blurred images or non-uniform image density. This tendency becomes more noticeable when the toner has a smaller particle diameter.
Such under-development and fog are presumed to be caused by an excessively low toner content (i.e., toner concentration) in developer or a poor rise for rapid triboelectric charging between the toner being fed and the carrier contained in the two-component type developer, where any uncontrollable, insufficiently charged toner thereby produced participates in development. It is essential for color developers to have the ability to always output images with a good image quality in the continuous copying of originals having a large image area. To deal with originals having a large image area and requiring a very large toner consumption, measures hitherto taken have more relied on improvements of developing apparatus than improvements of developers themselves. That is, it has been attempted to increase the peripheral speed of a developing sleeve or make a developing sleeve have a larger diameter so that the developing sleeve can be brought into contact with electrostatic latent images more times.
Such measures can be effective for improving developability, but may greatly limit the lifetime of apparatus because of an in-machine contamination due to toner scatter occurring in developing assemblies or because of an overload on the drive of developing assemblies. In some instances, measures are also taken in which developers are put in developing assemblies in large quantities in order to compensate the insufficiency of developability of the developers. Such measures, however, cause an increase in weight of copying machines, a cost increase due to the apparatus that must be made larger in size and an overload on the drive of developing assemblies as in the above case, and are not so much preferable.
Now, studies are reported on improvements made from both directions of toners and carriers for the purpose of maintaining a high image quality over a long period of running.
For example, Japanese Patent Applications Laid-open No. 51-3238, No. 58-144839 and No. 61-204646 suggest average particle diameter and particle size distribution of carriers. Of these, Japanese Patent Application Laid-open No. 51-3238 makes reference to a rough particle size distribution. It, however, has no specific disclosure as to magnetic properties closely concerned with developing performance of developers or transport performance thereof in developing apparatus. Moreover, carriers used in Examples all contain particles with a size of 250 mesh-pass or larger in an amount of as large as about 80% by weight or more and also have an average particle diameter of 60 .mu.m or larger.
Japanese Patent Application Laid-open No. 58-144839 only discloses average particle diameter of a carrier. It makes reference to the quantity of fine powder that influences the adhesion of carriers to photosensitive members and the quantity of coarse powder that influences the sharpness of images. It takes account of performance of color copying, and has no detailed disclosure as to particle size distribution of carriers. As for Japanese Patent Application Laid-open No. 61-204646, it discloses as the gist of the invention a combination of a copying machine with a suitable developer, and has no specific disclosure as to the particle size distribution or magnetic properties of carriers. It still also has no disclosure as to why such a developer is effective for copying apparatus.
Japanese Patent Application Laid-open No. 49-70630 has a disclosure relating to magnetic force of carriers, which, however, is concerned with iron powders used as carrier materials, having a larger specific gravity than ferrites, also having a high saturation magnetization. Such iron powder carriers have been hitherto put into wide use, but tend to make the weight of copying machines larger or cause an overload on drive torque, and also have a large environmental dependence.
A ferrite carrier disclosed in Japanese Patent Application Laid-open No. 58-23032 concerns a porous material with many voids. Such a carrier tends to cause the edge effect, having a poor durability, and has been found to be unsuitable for color copy carriers.
It has long been sought to provide a developer that enables continuous reproduction of images with a large image area, using a developer in a small quantity, and can satisfy the performance specific to color copying that no edge effect may occur even after running. Studies are made on developers and carriers, almost all of which, however, are proposed taking account of black and white copying, and only a little of which are proposed as those applicable also to full-color copying. It is also sought to provide a carrier having the ability to continue reproduction of images having an image area of 20% or more, which are nearly solid images, and having the ability to decrease the edge effect and retain the uniformity of image density on a sheet of reproduction.
Under such circumstances, the present inventors have proposed, as disclosed in Japanese Patent Application Laid-open No. 2-281280, a carrier with a narrow particle size distribution in which the presence of fine powder and the presence of coarse powder have been quantitatively controlled, to achieve a carrier improved in developing performance.
However, as previously stated, there is an increasing commercial demand for making copying machines achieve a higher minuteness and making images have a higher quality. In the present technical field, it is attempted to make toner particle diameter smaller so that a color image can be formed in a high image quality. Making the particle diameters of toner particles smaller results in an increase in the surface area per unit weight, tending to bring about an excessively large quantity of triboelectricity of the toner. This is accompanied with a possibility of the insufficiency of image density or the deterioration of running performance.
Thus, for the purpose of preventing the insufficiency of image density or the deterioration of running performance, caused by the toner made to have a smaller particle diameter, or for the purpose of improving development efficiency, it is attempted to make carrier particles have a smaller diameter. Such carriers, however, have achieved no quality high enough to stand against changes in the environment of toners or changes in the quantity of triboelectricity after running.